The present invention relates to a method and an apparatus for forming a multicolor image with a laser.
Laser printers using techniques of electrophotography are known as nonimpact printers. A laser beam is modulated in accordance with image data and is then deflected to divide a two-dimensional image into pixels or picture elements which are formed on a photoconductive drum. Thereafter, an image is recorded by a known electrophotography process. In this case, the two-dimensional image data is divided into pixels and is recorded in the form of pixels, so that merging of different types of data into single data, data deletion, and a change in character size can be easily performed.
It is important in image recording to display a multicolor image in order to easily distinguish one type of information from another. For example, the ability to distinguish between format information, mark information and document information, and between main information and additional or supplementary information, has been demanded.
In order to satisfy the above-mentioned demands, a method is considered in which a unicolor image is formed each time the photoconductive drum is rotated by one revolution. For example, if the operator wishes to obtain a three-color image, a single copy or print of the desired image can be obtained by rotating the photoconductive drum three times. However, the printing speed achieved using this method is slower than that in the case of unicolor printing. In fact, a three-color printer requires three times the printing time required by a unicolor printer. Furthermore, in the three-color printer, highly precise color registration is required, thus resulting in a complicated mechanism.
In order to solve the above drawback, an apparatus is disclosed in Japanese Patent Disclosure (KOKAI) No. 54-143139 in which a three-color image can be obtained by a single revolution of a photoconductive drum. More particularly, latent image formation and development for respective colors are sequentially performed on the photoconductive drum while the photoconductive drum is rotated by one revolution. However, in the apparatus of this type, a plurality of laser scanning systems (laser beam sources, optical systems, modulators, and deflectors) must be used, and the apparatus becomes complex as a whole, large in size and expensive to manufacture.
A two-color PPC process is disclosed in Japanese Patent Disclosure (KOKAI) No. 54-143139, in which a photoconductive drum having two photoconductive layers with different chromatic sensitivities is used. According to this process, the image exposure and transfer processes are performed once while the drum is rotated by one revolution. However, this prior art is essentially different from the present invention in that the photoconductive drum has a plurality of photoconductive layers. In this prior art, since the plurality of photoconductive layers are overlaid upon each other, much attention must be paid to the combination of photoconductive layers, thus disabling a wide variety of applications.